The present invention relates to a sealed prismatic battery having power generation elements accommodated in a battery case having a substantially prismatic cross section.
Recently, as a variety of portable electrical devices have been developed, the development of batteries for supplying drive power to the devices has been increasingly valued as a critical one of key devices. Among other things, compact rechargeable batteries such as nickel-metal hydride rechargeable batteries and lithium rechargeable batteries have been developed and increasingly grown in demand to be used recently in applications such as a power supply for driving hybrid electric vehicles in addition to cellular phones, notebook personal computers, or video cameras.
In recent years, electrical devices such as cellular phones have been increasingly demanded to be more compact and thinner, which has been in turn raising requirements for more compact and thinner batteries as their power sources. The present batteries are largely divided into cylindrical and prismatic ones in shape. For the cylindrical battery, a plurality of batteries may be accommodated in a pack case to form a battery pack or a plurality of batteries may be accommodated in the battery storage portion of an electrical device. In either case, poor space efficiency is provided due to the presence of many useless spaces or dead spaces as well as instability is caused inside the storage space. This makes the cylindrical battery unsuitable for the aforementioned electrical device that are to be made thinner and more compact. In contrast to this, the prismatic battery provides high space efficiency and can be accommodated with stability in the storage space, thus being suitable for the electrical device that are to be made thinner and more compact.
However, the sealed prismatic rechargeable battery may be subjected to an increase in pressure inside the battery due to gases produced during charge and discharge, and expansion of the power generation elements accommodated in the battery case. In this case, the battery case is deformed to take a more stable shape, that is, a circular shape, such that long lateral walls of the battery case are subjected to a relatively large deformation to expand outwardly. Such a deformation of the battery case causes various deleterious effects. For example, in a sealed battery where the outermost electrode plate of a spiral-wound electrode group is in electrical contact with the inner surface of the battery case, the contact resistance between the outermost electrode plate and the battery case is increased, resulting in an increased internal resistance of the battery.
In this context, conventionally suggested are the following methods of providing means for preventing the battery case from being deformed due to an increase in pressure inside a prismatic battery. Japanese Patent Laid-Open Publication No. Sho. 62-93854 discloses providing part of a battery case with a portion having an increased thickness, thereby reinforcing the battery case. Japanese Patent Laid-Open Publication No. Hei. 7-326331 discloses a battery case having a rectangular cross section, configured such that the inner surface of each corner is gradually arc-shaped to make the corners thicker than the long and short lateral walls, which are connected to each other at each corner.
By providing part of the battery case with a portion having an increased thickness, the battery case can be prevented from being deformed due to an increase in pressure inside the battery. However, the battery case is increased in outer shape and thus cannot be made compact. On the other hand, by increasing the thickness at the corners of the rectangular battery case, the battery case can be prevented from being deformed due to an increase in pressure inside the battery while preventing the battery case from increasing in outer shape. However, the battery case is reduced in volume and thus filled with a reduced amount of active substances for positive and negative electrodes of the electrode group, thereby reducing the energy density per volume.
The present invention has been developed in light of the aforementioned problems. An object of the present invention is to provide a prismatic battery which has a battery case that causes neither an increase in outer shape nor a decrease in inner volume but has a high pressure resistance enough to prevent the battery case from being deformed due to an increase in pressure inside the battery.
To achieve the above object, the present invention provides a prismatic battery having a prismatic tubular battery case for housing an electrode group and electrolyte solution, with an opening of the battery case being sealed with a sealing member. The prismatic battery is characterized in that the battery case, substantially square in cross section, has four lateral walls each being gradually arc-shaped to curve outwardly with a radius of curvature R1, and in that the cross section satisfies equation R1/r=4 to 20, where r is the distance between a midpoint of each lateral wall and the center point in the cross section of the battery case and R1 is the radius of curvature.
The prismatic battery is configured to have the battery case substantially square in cross section with each lateral wall of the battery case being slightly expanded outwardly to satisfy that R1/r greater than 4, and thus has high space efficiency generally equal to that of existing prismatic batteries. In addition, the prismatic battery is configured such that the lateral walls of the battery case are gradually arc-shaped to expand outwardly to satisfy that R1/r less than 20. When an increase in pressure inside the battery occurs due to gases produced during charge and discharge and expansion of the electrode group, this configuration positively prevents each lateral wall from being deformed to further expand outwardly. Moreover, since this prismatic battery increases its inner volume by such an amount as provided by the lateral walls that are slightly expanded outwardly, it is possible to accommodate more power generation elements such as electrolyte solution in the increased space, thereby improving the energy density. In addition, with the battery case being entirely formed in a uniform thickness, the battery case is free from any decrease in inner volume. Even in generally the same outer shape as that of an existing prismatic battery, the prismatic battery provides a large amount of current at the time of charge and discharge. This allows the prismatic battery to provide the battery characteristics of a large capacity and a high energy density.
The invention also provides a prismatic battery having a prismatic tubular battery case for housing an electrode group and electrolyte solution, with an opening of the battery case being sealed with a sealing member. The prismatic battery is characterized in that the battery case, substantially rectangular in cross section and with a pair of long lateral walls facing each other and a pair of short lateral walls facing each other, has the cross section such that the long lateral walls are each gradually arc-shaped to curve outwardly with a radius of curvature R11 to satisfy equation R11/r1=4 to 20, where r1 is the distance between a midpoint of the inner surface of the long lateral wall and the center point in the cross section of the battery case and R11 is the radius of curvature. The prismatic battery is also characterized in that the short lateral walls are each gradually arc-shaped to curve outwardly with a radius of curvature R12 to satisfy equation R12/r2=4 to 20, where r2 is the distance between a midpoint of the short lateral wall and the center point in the cross section of the battery case and R12 is the radius of curvature.
The prismatic battery is configured to have the battery case substantially rectangular in cross section with the long lateral walls of the battery case being slightly expanded outwardly to satisfy that R11/r1 greater than 4 and with the short lateral walls being slightly expanded outwardly to satisfy that R12/r2 greater than 4, and thus has high space efficiency generally equal to that of existing prismatic batteries. In addition, the prismatic battery is configured such that the long lateral walls of the battery case are gradually arc-shaped to expand outwardly to satisfy that R11/r1 less than 20 and the short lateral walls are gradually arc-shaped to expand outwardly to satisfy that R12/r2 less than 20. When an increase in pressure inside the battery occurs due to gases produced during charge and discharge and expansion of the electrode group, this configuration positively prevents each lateral wall from being deformed to further expand outwardly. Moreover, since this prismatic battery increases its inner volume by such an amount as provided by the lateral walls that are slightly expanded outwardly, it is possible to accommodate more power generation elements such as electrolyte solution in the increased space, thereby improving the energy density. In addition, with the battery case being entirely formed in a uniform thickness, the battery case is free from any decrease in inner volume. Even in generally the same outer shape as that of an existing prismatic battery, the prismatic battery provides a large amount of current at the time of charge and discharge. This allows the prismatic battery to provide the battery characteristics of a large capacity and a high energy density.